1. Field of the Invention
The present invention pertains to the removal of metals, nitrogen, phosphorus, oxygen-demanding substances, and certain classes of synthetic organic compounds from municipal, industrial, urban, and agricultural wastewaters and runoff. In addition, the invention has application to the treatment of surface and ground waters.
2. Description of Related Art
Chemicals such as iron and aluminum salts frequently are used to coagulate and remove phosphorus and other contaminants from wastewaters and runoff. One drawback of chemical treatment technologies, besides the expense, is that most of them contribute toxic residual components (e.g., aluminum, caustic pH, and a potential for sulfide production) either to the treated wastewater or to the residual sludge/backwash. Chemical treatment systems also require operator control and maintenance, coagulant aids, pH stabilization, the use of hazardous chemicals, and sludge/backwash residuals disposal, all factors that are expensive and require energy supplements and process control capability. The cumulative effect of these requirements is that the systems are reliable only when they are designed, constructed, maintained, and operated by skilled personnel. For instance, in the case of phosphorus, there are instances when the target 50 ppb total phosphorus (TP) concentration in treated agricultural drainage waters has not been met with chemical-intensive techniques, even with intensive oversight (Anderson et al., 1992).
Wetlands, artificial (constructed) or natural, are commonly used as a less expensive and more passive vehicle than conventional technology for treating many types of wastewaters, such as domestic sewage, urban and agricultural runoff, industrial and mining wastes. Wetlands can support and provide the necessary biogeochemical processes needed for the transformation, reduction, and immobilization of primary, secondary, and trace pollutants (including xenobiotics and heavy metals). The passive nature of wetland treatment technologies makes them cost effective compared to more traditional engineered wastewater treatment systems.
One drawback to wetland treatment systems is their requirement for large areas of land, approximately 10-100 times as much land area as a conventional treatment plant, or, in terms of capacity, 5-40 acres per 10.sup.6 gallons of wastewater treated per day. Another serious drawback in existing wetland wastewater treatment technologies has been the poor removal performance of some contaminants, such as total phosphorus (Dierberg and Brezonik, 1983; Richardson, 1985). Long-term TP removal in wetlands underlain by organic soils such as in the Everglades relies on peat accumulation, which occurs at a rate of approximately 1 g P/m.sup.2 year. Although this is an extremely slow process, given enough wetland area, concentrations of TP in sugarcane field drainage water can be reduced to less than 50 ppb (Abtew et al., 1995).
Limerock beds or filters have been used less frequently for removing contaminants from surface and wastewaters than have wetlands. Whereas physical (e.g., settling, filtration) and biological (e.g., oxidation, nitrification, denitrification) processes dominate in attenuating contaminants in wetlands, chemical processes (e.g., adsorption, surface complexation, surface precipitation) predominate in the removal of contaminants on the surfaces of limerock.
To date, most of the applications of limerock for contaminant removal in waste streams have been experimental, often producing lackluster results. This is because of the moderate pH typically found for waters in equilibrium with calcium carbonate; pH values of phosphorus-laden waters are frequently lower than 8.0 due to the respiratory activities of microbiota and the adsorption/precipitation reactions, both of which produce hydrogen ions. It has been found that the efficacy of contaminant removal mediated by limerock increases with pH from a value of approximately 8.0, below which the process is minimally effective.